Chemical vapor deposition and plasma-enhanced chemical vapor deposition are processes that are widely used to fabricate semiconductor devices.
Chemical vapor deposition (CVD) is a chemical reaction in which gaseous molecules called precursors are transformed into a thin film on the surface of a substrate, such as an application-specific integrated circuit (ASIC) wafer.
Plasma-enhanced chemical vapor deposition (PECVD) is a similar process which occurs in a reactor wherein a strong electric field causes decomposition of a gaseous compound into a plasma (of highly ionized gases). This reaction leads to precipitation of a film onto the substrate surface.
CVD and PECVD processes for the fabrication of semiconductor devices typically use amorphous silicon rather than crystalline silicon. Crystalline silicon naturally exists as a lattice structure in which each silicon atom is tetrahedrally bonded to four neighboring silicon atoms. Amorphous silicon (a-Si) is the “non-crystalline” form of silicon. Amorphous silicon is replete with defects known as “dangling bonds”, i.e., silicon atoms that are not tetrahedrally bonded. Amorphous silicon is also referred to as “undercoordinated” because not all of its atoms are “four-fold coordinated” with four neighboring atoms.
For fabrication of semiconductors, however, pure amorphous silicon has been found to contain too many defects (i.e., too many dangling bonds). Because pure amorphous silicon has such a high density of defects (dangling bonds), trapped charges have a proclivity to recombine. Recombination is a phenomenon whereby a free electron traveling in the vicinity of a hole forms a covalent bond, thus reducing the number of charge carriers. Therefore, to limit the likelihood of recombination, engineers and other persons skilled in the art have resorted to hydrogenated amorphous silicon (a-Si:H). The electrical properties of hydrogenated amorphous silicon are superior to those of pure amorphous silicon because the former contain fewer dangling bonds.
One way to control the electrical properties of a semiconductor is to vary the number of charge carriers. The process of doping, which is well known in the art, is a commonly used way of introducing impurities in order to control the number of charge carriers in the semiconductor. Usually, semiconductors are doped to increase the number of charge carriers and thus to increase the conductivity of the semiconductor.
In general, there are two types of doping. N-type doping involves doping with a donor element which donates a free electron to the lattice structure of the semiconductor. The resulting semiconductor is known as an N-type semiconductor P-type doping involves doping with an acceptor element which removes an electron to make a hole in the structure. The resulting semiconductor is known as a p-type semiconductor.
For CVD and PECVD film deposition processes, doping is also known in the art as a technique to the modify the electrical properties of the film. For n-type doping, a dopant such as phosphine (PH3) may be used. For p-type doping, a dopant such as diborane (B2H6) may be used. These dopants are added to the process gas in order to introduce elementary phosphor or boron, as the case may be, into the new silicon film.
One of the shortcomings of semiconductor films made of deposited amorphous silicon is that the film is often too conductive or does not possess a sufficiently high breakdown voltage to be useful in certain applications where these electrical properties are either necessary or desirable. Accordingly, a method of fabricating a semiconductor film that reduces the conductivity and/or increases the breakdown voltage of the deposited film remains highly desirable.